First tried out MeeMix shortly after moving from Halifax to Toronto (early months of 2007), discovering that they'd finally started cracking down on non-US IPs, and saw an ad for it on Facecrook.
Here's the thing: MeeMix has some pretty cool interface options that I wish Pandora had. I couldn't give a toss about the social aspects of MeeMix--I don't care who listening to the same bands that I've never met--but I really like their sliding scale of like/dislike of a song. Pandora's very... polar about it. Thumbs up, meh, or die-in-a-fire. I really prefer being able to say anything between "This is awesome, I want to hear more like this", "This is pretty cool, I like it", "I'm not a huge fan, but it's not terrible" and "Seriously?!" before getting to die-in-a-fire.
That being said, Pandora's been trained quite well. I have an entire station of nothing but early-to-mid-nineties alt rock (I can listen at work, I don't know why). It's great, it reminds me of when music was good. MeeMix hasn't been trained nearly so well, but they also introduced me to Fluke and Sparta.
I say, if you miss Pandora, go with MeeMix. Takes a bit of doing to train it as well as Pandora, but they've got some pretty good stuff in there that might surprise you. And the eleven-point scale of like/dislike (and that doesn't even include the "I never want to hear this again" option) is great.
That being said.. I really wish I could get Pandora at home.